More and more functions in modern vehicles are realized by electrical components. This creates rising demand for electrical power. In order to satisfy this demand, the efficiency of the generator system in the motor vehicle must be increased. As a rule, silicon diodes are currently used as Z-diodes in the generator system of motor vehicles. Advantages of the cost-effective silicon diodes are their low blocking current and their great robustness.
One disadvantage of silicon diodes is the relatively high forward voltage FV. At room temperature, the current begins to flow only at a FV=0.7V. Under normal operating conditions, e.g., at current densities of 500 A/cm2, the FV rises to more than 1V. These forward power losses reduce the efficiency of the generator considerably. Another disadvantage of silicon diodes is the positive temperature coefficient of the breakdown voltage.
The breakdown voltage of silicon diodes is defined by the avalanche generation and increases with rising temperatures. If Z-diodes are used for limiting the vehicle system voltage, it may therefore no longer be possible to ensure the protective function at high ambient temperatures and special operating conditions (reduced load, load dumping). The voltages in the vehicle electrical system then rise beyond the maximally tolerated value for a brief period of time and lead to damage to electronic components that obtain their supply voltage from the vehicle electrical system.
To reduce the forward power losses, the DE laid-open document 102004056663 refers to the use of so-called high-efficiency diodes (HED) instead of silicon diodes. High-efficiency diodes (HED) are a new type of Schottky diodes, which, in contrast to conventional Schottky diodes, exhibit no barrier-lowering effects caused by the blocking voltage, and therefore have low blocking currents. High-efficiency diodes (HED) are made up of a combination of conventional Schottky diodes with other elements such as magnetoresistive elements, pn-junctions or different barrier metals, the combination being monolithically integrated on a semiconductor chip. They are frequently implemented in trench technique. A HED then includes at least a few trench structures. The trenches are approximately 1-3 μm deep and approximately 0.5-1 μm wide. The use of HEDs makes it possible to realize considerably lower forward voltages FV of approximately 0.5-0.6V.
An alternative to HEDs is the heterojunction diode or HJD referred to in DE laid-open document 102006024850. In contrast to conventional pn-junctions, in which the two differently doped layers are made from the same semiconductor material, e.g., silicon, a heterojunction is formed by a p-doped layer of silicon germanium (Si1-xGex), for example, and an n-doped layer of silicon (Si). The index “x” denotes the germanium component in this case. For example, x=0.3 corresponds to a Germanium component of 30%.
One exemplary embodiment of a hetero-junction diode HJD is shown in FIG. 1. The illustrated HJD is made up of an approximately 200 μm thick, highly n-doped silicon substrate 1. Above it is an n-doped silicon epitaxial layer 2 having a thickness of approximately 1.1 μm and a doping concentration of 4.5×1016 1/cm3, for example. Above this layer is SiGe layer 3 having a germanium component of 10-40%. This SiGe layer is approximately 10-50 nm thick and doped with boron at a concentration of >1019 1/cm3. At higher dopings, a stepped p-doping profile is advantageous.
Both the SiGe layer 3 at the topside and silicon substrate 1 at the underside of the chip are provided with metal contacts 4 and 5. These contacts may be made up of a sequence of coatings of chromium, nickel and silver, for instance. Contacts 4 and 5 form the anode and cathode electrodes of the diode. Using heterojunction diodes HJDs makes it possible to achieve forward voltage FV that are smaller than in a conventional diode made from one semiconductor material only.
In contrast to the HEDs referred to in DE laid-open document 102004056663, which are made up of a multitude of very fine structures (<μm), HJDs are easier to produce. The energy barrier of heterojunctions is markedly less dependent on the applied blocking voltage since the characteristic barrier-lowering effects of Schottky diodes do not arise. As a consequence, the blocking currents in HJDs are lower than in conventional Schottky diodes, even without involved measures that must be undertaken in the case of the HED, for example.
An edge structure is required both for conventional silicon diodes and also for the above-mentioned alternatives of HEDs or HJDs. Without edge structure the field intensity at the surface at the chip edge would be higher than in the interior of the chip. This would cause the breakdown to occur already below the desired voltage and to take place in an area that is far too small (excessively high current density!) at the edge of the chip. The field intensity in the edge area of the component is reduced by a suitable edge structure. Thus, the breakdown no longer occurs in the edge region but in the center of the component. One exemplary embodiment of an edge structure are the so-called floating guard rings.
FIG. 2 shows a pn-diode having a guard ring edge structure. As can be gathered from FIG. 2 using the example of a silicon diode, the inner structure of this diode is made up of a highly n-doped silicon substrate 1, a superposed n-doped silicon epitaxial layer 2, and at least one p-doped trough 6 diffused into n-silicon epitaxial layer 2. Metal contacts 4 and 5 form the anode and cathode electrodes of the diode, respectively. The edge structure is made up of at least one circumferential p-doped trough 66 diffused into n-silicon epitaxial layer 2. An oxide layer 7 is situated above the edge region of the component in order to protect the silicon surface from electrical shortcuts as well as from contaminations of different types. Trough 66 is provided in order to expand the space charge region in the edge region so that the field intensity in the edge region of the component is reduced. The breakdown voltage of the diode is then defined not by the edge region but by the center of the diode. The function of trough 66 does not consist of carrying high currents, which is why the circumferential trough usually has relatively narrower dimensions and is called a “ring”.
Another example of a pn-diode having an edge structure is the pn-diode having the magnetoresistor edge structure shown in FIG. 3. FIG. 3 also shows a silicon diode made up of the same elements as in FIG. 2: a highly n-doped silicon substrate 1, a superposed n-doped silicon epitaxial layer 2, at least one p-doped trough 6 diffused into the n-silicon epitaxial layer, and a metal contact 5, which once again serves as cathode electrode of the diode. Metal layer 44, which is used as anode electrode of the diode, extends beyond p-doped trough 6 at the edge. An oxide layer 7 is situated between metal 44 and p-doped trough 6 or n-doped silicon epitaxial layer 2. This metal-oxide-Si structure is what is referred to as a magnetoresistive element. It also has the task of expanding the space charge region in the edge region so that the field intensity is reduced in the edge region of the component.
A metal ring 8, which prevents the space charge zone from reaching the chip edge, is situated at the edge of the chip. Such a magnetoresistive-element edge structure also makes it possible for the breakdown to take place in the center of the component. In addition to the two mentioned examples, still other edge structures are known, of course. A shared characteristic is that these structures require additional chip area. Moreover, further process steps or masks are frequently required as well. This entails a higher defect risk. All in all, this results in higher production costs.
Additional structures at the surface of the chip edge also pose a certain quality risk during operation of the components, because both the top surface and also the chip edge are exposed to contamination and mechanical stress to an especially high degree.